Owing to its poor electrical conductivity, polyethylene, like other polymeric materials, becomes electrostatically charged. This property stands in the way of its use in various technical fields and has led to the development of materials which are antistatic, i.e. possess at least a low electric conductivity. Solid materials which are classified as antistatic are those having a surface resistance of at most 10.sup.9 .OMEGA..
Numerous measures are known for preventing the electrostatic charging of polyolefins. The conductivity can be achieved, as early as in the synthesis, by chemical modification of the plastic itself by influencing its molecular structure. Subsequently, it can be obtained, for example, by oxidation of the surface or addition of electrically conductive fillers (antistatic agents). Suitable antistatic agents are ionogenic organic compounds of differing constitution having a high boiling point, which compounds are sufficiently compatible with the polyolefin, i.e. do not tend to sweat out. These include substances containing hydroxyl, amino, or amide groups. For certain applications, metal fibers, platelets, or powders can also be used as antistatic fillers.
The classical antistatic agent is carbon black which, above a minimum concentration of between 5% and 10%, produces electrical conductivity in a wide variety of plastics. To effectively make polyethylenes having viscometrically determined mean molecular weights up to about 500,000 g/liter antistatic, a uniformly distributed carbon black content of from 10% to 20% by weight is required. The disadvantage is that, at such relatively high concentrations, valuable properties of the polyethylene, e.g. its toughness and wear resistance, are adversely affected.
One possible way of avoiding the loss in quality associated therewith is to distribute the filler non-randomly in the specified polyethylene. Such a distribution is achieved by use of conductive filler particles having a substantially smaller diameter than the diameter of the primary particles of the plastic. Mixtures of this composition are then pressed at temperatures at which complete melting of the plastic does not occur; this prevents the electrically conductive filler from migrating into the polymer particle. The result is the formation of a core-shell structure which, compared to the homogeneous distribution of carbon black, gives the same electric conductivity at significantly lower carbon black content. To obtain the core-shell structure at least partially, the particles must not be completely melted and, for this reason, the structure can only be obtained by pressing.
A proven process which gives polyethylenes of the ultra high molecular weight (UHMWPE) type is the subject matter of DE-C-2 361 508. It is carried out at pressures of 0.1 to 10 MPa and temperatures of 30.degree. to 130.degree. C. using catalysts comprising titanium (III) halides and organic aluminum compounds. Other processes are also suitable, e.g. ethylene polymerization in the presence of chromium oxide catalysts which likewise proceeds at low pressures.
UHMWPE has a group of physical properties which makes it suitable for a wide variety of applications. Particular mention should be made of its high wear resistance, its low coefficient of friction with other materials, and its excellent toughness. In addition, it is remarkably resistant to numerous chemicals.
Owing to this favorable tribological and chemical behavior, UHMWPE is used in a wide variety of technical fields as a multi-faceted material. Examples are the textile industry, machine construction and the chemical industry. However, there is the disadvantage that UHMWPE, owing to its extremely low flowability even above the crystallite melting point, can be processed into shaped articles only with difficulty using methods customary for thermoplastics. Thus, for example, processing of UHMWPE on plastic-processing machines suitable for low molecular weight polyethylene (LMWPE) does not enable production of moldings in which the excellent mechanical properties of the starting material remain unaltered.